The present invention relates to a transfer case for controlling the distribution of drive torque between the front and rear drivelines of a four-wheel drive vehicle.
Due to increased consumer demand for four-wheel drive vehicles, a variety of different transfer cases are currently utilized for directing power (i.e. drive torque) to the front and rear drivelines of the vehicle. For example, many xe2x80x9cpart-timexe2x80x9d transfer cases are normally operable in a two-wheel drive mode for delivering drive torque to the driven wheels. When four-wheel drive is desired, a xe2x80x9cmodexe2x80x9d shift mechanism can be selectively actuated by the vehicle operator for directly coupling the non-driven wheels to the driven wheels for establishing a four-wheel drive mode. As will be appreciated, motor vehicles equipped with such a part-time transfer case offer the vehicle operator the option of selectively shifting between the two-wheel drive mode during normal road conditions and the four-wheel drive mode for operation under adverse road conditions. An example of such a transfer case is disclosed in commonly-owned U.S. Pat. No. 4,770,280 to Frost.
Alternatively, it is known to use xe2x80x9con-demandxe2x80x9d transfer cases for automatically directing power to the non-driven wheels, without any input or action on the part of the vehicle operator, when the traction is lost at the driven wheels. Modernly, the xe2x80x9con-demandxe2x80x9d feature is incorporated into the transfer case by replacing the mode shift mechanism with a clutch assembly that is interactively associated with an electronic control system and sensor arrangement. During normal road conditions, the clutch assembly is maintained in a non-actuated condition, such that drive torque is only delivered to the driven wheels. However, when the sensors detect a low traction condition at the driven wheels, the clutch assembly is automatically actuated to deliver drive torque xe2x80x9con-demandxe2x80x9d to the non-driven wheels. Moreover, the amount of drive torque transferred through the clutch assembly to the non-driven wheels can be varied as a function of specific vehicle dynamics, as detected by the sensor arrangement. One example of such an xe2x80x9con-demandxe2x80x9d transfer case is disclosed in commonly-owned U.S. Pat. No. 5,363,938 to Wilson et al, wherein the electronically-controlled clutch assembly is operable for automatically controlling the amount of drive torque transferred to the non-driven wheels as a function of the wheel speed difference (i.e. the wheel slip) between the driven and non-driven wheels.
As a further alternative, some vehicles are equipped with full-time transfer cases having an inter-axle differential that functions to permit inter-axle speed differentiation while transferring drive torque to both the front and rear drivelines. To minimize loss of traction due to wheel slippage, many full-time transfer cases are also equipped with a clutch assembly for limiting speed differentiation and biasing the torque transferred across the center differential. For example, full-time transfer cases are disclosed in commonly-owned U.S. Pat. No. 5,697,861 to Wilson and U.S. Pat. No. 5,702,321 to Bakowski et al, which are respectively equipped with mechanically-actuated and hydraulically-actuated clutch assemblies for controllably modulating the drive torque delivered across the center differential to the front and rear drivelines.
Typical transfer cases, like those discussed above, are capable of providing both low and high output drive ratios. This is generally achieved through use of a planetary gear set, the components of which are selectively engaged by a sliding dog clutch for establishing the desired ratio. Generally, the output drive ratio is selected using a range shift mechanism that is operable to move the dog clutch either manually by movement of a driver-operated shift lever or electronically at the driver""s signal by a power-operated actuator. The above-referenced commonly-owned U.S. Patents illustrate two-speed transfer cases equipped with a range shift mechanism.
While the transfer cases disclosed above have advanced the technology associated with four-wheel drive motor vehicles, a need still exists to develop alternative arrangements which continue to provide improvements in packaging, response time and cost. One particular focus for improvement is the range shift mechanism.
The present invention provides an improved power transfer system for use in four-wheel drive vehicles having a two-speed transfer case equipped with a planetary gearset and a range shift system for selectively establishing low and high output drive ratios.
In accordance with one object of the present invention, a transfer case includes an input shaft, a first output shaft, a second output shaft, a planetary gearset, an interaxle differential, and a range shift system having a first clutch mechanism and a second clutch mechanism. The planetary gearset includes a sun gear driven by the input shaft, an annulus gear, and a plurality of pinion gears rotatably mounted on a carrier and meshed with the sun gear and the annulus gear. The interaxle differential has an input driven by the carrier, a first output driving the first output shaft and a second output driving a transfer mechanism coupled to the second output shaft. The first clutch mechanism is operable in a released state to permit rotation of the annulus gear and is further operable in an engaged state for braking the annulus gear. The second clutch mechanism is operable in an engaged state for coupling the input shaft for rotation with the carrier and is further operable in a released state for permitting relative rotation between the input shaft and the carrier.
As a related object of the present invention, the first clutch mechanism is a first bi-directional overrunning clutch having a first actuation member moveable between a first position and a second position. A spring normally biases the first actuation member toward its first position whereat the first bi-directional overrunning clutch is released to permit rotation of the annulus gear. A clutch actuation system is provided for selectively causing movement of the first actuation member to its second position whereat the first bi-directional overrunning clutch is engaged to prevent rotation of the annulus gear.
A further object of the present invention provides that the second clutch mechanism is a second bi-directional overrunning clutch having a second actuation member moveable between a first position and a second position. A spring normally biases the second actuation member to its first position whereat the second bi-directional overrunning clutch is engaged to prevent relative rotation between the carrier and the input shaft. The clutch actuation system is further operable for selectively causing movement of the second actuation member to its second position whereat the second bi-directional overrunning clutch is released to permit the carrier to rotate relative to the input shaft.
Yet another object of the present invention is to provide a mode clutch between the carrier and one of the first and second outputs of the interaxle differential and which is controlled by the clutch actuation system for modulating torque bias and limiting slip across the interaxle differential.
According to yet another object of the present invention, the clutch actuation system is electronically-controlled to permit xe2x80x9con-the-movexe2x80x9d range shifts and selection having the vehicle operator of various available drive modes.
In accordance with an alternative embodiment of the present invention, the first clutch mechanism is substituted by a band brake disposed about the annulus gear and which can be selectively actuated to brake rotation of the annulus gear.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are intended for purposes of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.